The concept of a ring-down cavity to perform spectroscopy has been developed for application in the visible and infrared portions of the spectrum and discussed in the technical literature. Cavity ring-down (CRD) spectrometers may generally be one of two types, namely pulsed and continuous wave. In the pulsed case, a short pulse of light may be injected into a resonant cavity (often referred to as a cavity resonator or sample cell) which may include a pair of mirrors between which light may reflect. In spectrometer applications, the space between the mirrors in these cavities may be filled with a sample medium so that the absorption spectrum of the sample can be measured.
The mirrors may not be perfectly reflecting but may allow some light to pass through the mirrors for entrance and exit of the light. If a short single pulse is injected into an empty cavity, the pulse may reflect many times in the cavity, and on each encounter with an exit mirror, a pulse may exit the cavity. Thus, a single pulse may produce a train of pulses with each pulse subsequently reduced in pulse height as energy leaks out of the exit mirror on subsequent passes. The multiple reflections may be referred to as “ringing,” and the time it takes the output pulse train to complete may be referred to as the “ring-down time.” This time constant is a property of the cavity geometry and the medium between the reflecting mirrors. For more information on a pulsed ring-down cavity, see for example, K. Lehmannn & D. Romanini, The Superposition Principle and Cavity Ring-Down Spectroscopy, J. CHEM. PHYS., vol. 105 no. 23 (1996) (hereinafter “Lehmannn & Romanini”), the content of which is hereby incorporated by reference in its entirety.